Artificial muscles--actuators based on such materials as certain types of metals and polymers that shrink, grow, or change shape--are useful for prosthetic limbs, microscale machines, and robots.
Several issues still must be corrected. As greater loads are applied to actuators, they can start to exhibit "creep"--that is, they do not completely return to their original state with successive cycles. Baughman says that before these actuators can be useful, creep must be eliminated. "Under load, the cycle is not reversible--you've got a little creep. In most actuator applications, you don't want any creep."
Another key issue is scaling up from thin individual threads. Although the carbon-nanotube muscles can outperform natural muscles on a per-area basis, exerting 100 times the force, natural muscles are much larger, making them stronger.